Random numbers are used in a variety of application such as computer games. But they are also used in more serious applications, for example in generating cryptographic keys and for some classes of scientific experiments. There are two species of random number generators, namely, software or pseudo random number generators and hardware random number generators. While the former is relatively inexpensive and easy to implement, the latter if properly designed, can provide very close to true randomness, but at a higher cost.
Software random number generators produce random numbers by using mathematical formulas or they are simply taken from a pre-calculated list. Extensive research has been devoted to software random number generators and there are presently algorithms available which are capable of generating random numbers which seem as if they are truly random. However, software random number generators have the characteristic that they are predictable.
Hardware random number generators, on the other hand, can be very close to truly random. They usually come in the form of an electronic device, although a container containing a number of numbered balls, such as that used in a lottery, also constitute a hardware random number generator. Hardware random number generators typically use an entropy source which exhibits randomness. For example, a common hardware random number generator used in a computer uses the random variations in a user's mouse movements or the amount of time between key strokes as an entropy source. Other hardware random number generators include those that use atmospheric noise from a radio and those that use a radioactive source and the points in time at which the radioactive source decays are used to generate random numbers.
There is another class of random number generator which is known as a hybrid random number generator. A hybrid random number generator uses a software algorithm in order to generate a pseudo random number. The output of a hybrid random number generator tends to approach a more random state when the input to the software algorithm is seeded with a random seed value. Such a random seed value is often obtained from a traditional hardware random number generator. As such, the hybrid random number generator is able to provide a much more random output than a true software random number generator.
One problem with either the hardware random number generator or a hybrid random number generator that relies on a hardware random number generator for a seed value is that a hardware random number generator may not always be able to provide a random number. Consider, for example, a hardware random number generator that is based upon the movement of a user's mouse. In the case where a user is not using the mouse, the hardware random number generator no longer has access to an entropy source. The same is true for a number of variations of hardware random number generators. In order to provide a continuous stream of random numbers, a suitable entropy source must be provided and must be readily accessible at any point in time.